1. Field of the Invention
This invention relates to error diffusion in printing, more particularly to error diffusion in color printing.
2. Background of the Invention
In digital printing, the output to the printing system that forms the necessary dots is binary, either ON or OFF. A comparison of the intended value to a threshold determines whether the output i s ON or OFF. For example, consider an 8-bit system with a range of possible values from 0-255. The threshold for ON or OFF is set to 127.5, or at the mid-point. Values above will be is turned ON, values below OFF.
However, the value given an ON dot will be 255, and OFF will be 0. Therefore, an error exists between the intended value and the printed value of the dot. For example, if a dot has an intended value of 200, it lies above the threshold and will be turned ON. Its printed value will be 255, so there is an error of +55 between its intended and printed values.
This error is typically handled with error diffusion, originally proposed by Floyd and Steinberg. The error value can be assigned to the next dot, since the dots are so relatively small in the perceived human eye response; the eye will tend to integrate the next several dots into one larger spot of color. Floyd and Steinberg suggested using four yet to be processed pixels that neighbor the pixel with the error value.
The Floyd-Steinberg approach 10 is shown in FIG. 1. The threshold is applied to the input color value, which in this example is 200. The threshold is 128, so that dot is printed. The resulting difference between the intended value of 200 and the printed value is +55. It will be diffused to the neighbors with the weights of 7 5 3 1, in the positions shown at 14. The sum of the weights is 16, so the error valued ed=55/16=3.44. The following table demonstrates the result error diffusion on the neighboring pixels. The original value for all pixels is 144.
It must be understood that the error values computed for each pixel are only those from the previous pixel. Other error values exist for the pixels from other neighboring pixels. For example, if X in the below diagram is the pixel with value 200 from the example, Y is the neighboring pixel. The pixel that has the 5 weight from X is the pixel that has the 3 weight from Y. The error diffusion for X is in parentheses ( ), the error diffusion for Y in the brackets [ ].
As mentioned above, the flowchart for this process is shown in FIG. 1. One problem with this method of error processing is directional hysteresis because of the scan line format. The error diffusion can lead to objectionable artifacts in the image. A significant artifact is a texture pattern in the mid-tone region.
These artifacts are especially noticeable in the color printing area. A perceived colored pixel is actually an integrated image in the eye that takes four separate dots that are so close together that they are resolved as one. Some attempts have been made to layer the different color dots on top of each other, but these are not usually successful. The problem with layering the dots on top of each other is that the slightest slip in registration between the colors leads to further artifacts.
Several solutions to this problem have been proposed. The mid-tone patterns can be deconstructed. Instead of printing the dots on top of each other, the dots can be offset slightly from the centroid of the pixel position. The human eye resolves it as one pixel. Alternatively, a good relationship can be established between the four color dot positions relative to the centroid of the pixel. However, even these techniques can leave artifacts in the image, especially in the regions with values between 64 and 192.
Therefore, a new technique is needed for artifact correction in error diffusion for the mid-tone regions of print images.
One aspect of the invention is a method of error diffusion in printing that uses at least two thresholds applied along directional guided threshold lines. The priority to print a dot for any given color depends upon its relationship to the threshold for that position and that color. The thresholds for each color are established along a different direction from the other colors. The series of thresholds is then assigned to the lines in the proper direction for that color. The input color value is compared to the threshold for that position and the priority is determined.
In one embodiment of the invention, three threshold values are used. The four color threshold line orientations use these three thresholds to determine the printing priority for that color at each position.